Curing of coatings through ultraviolet (UV) radiation requires efficient methods of initiating the chemical reaction responsible for the curing process. Cross-linking of polymeric material through generation of radical species upon irradiation with UV light is widely used to produce hydrogels for medical device coatings. Also, the paint and lacquer industry makes use of UV initiated curing of acrylates, where photoinitiators in many cases are employed. These two examples illustrate the diversity of UV curable coatings, where up until now the UV active component in the coating recipe relies on molecules with comparable low molecular weight. The UV active components are partially free to diffuse to the surface of the cured material thereby rendering these substances exposed to the environment. Higher molecular weight photoinitiators, in particular polymeric photoinitiators, have comparably higher intrinsic viscosities which most likely result in longer diffusion times through a matrix. Migration of the UV active substances to the surface is therefore diminished when polymeric photoinitiators are used as opposed to lower molecular weight photoinitiators. Scarce literature found in scientific and patent publications within the topic of polymeric photoinitiators suggests that development of such polymers could lead to novel applications and present solutions for existing needs, such as provide a material with negligent migration of substances to the surface. Available literature discussed in the following, outlines previous work within the field of polymeric photoinitiators, with focus on work relevant for photoinitiators with a polyalkylether backbone.
Some descriptions of polymeric photoinitiators are found in scientific literature, where for example 4-amino-4′-[4-aminothiophenyl]benzophenone is polymerized with toluene-2,4-diisocyanate (J. Wei, H. Wang, J. Yin J. Polym. Sci., Part A: Polym. Chem., 45 (2007), 576-587; J. Wei, H. Wang, X. Jiang, J. Yin, Macromolecules, 40 (2007), 2344-2351). Examples of the use of this photoinitiator to polymerize acrylates are also given in this work. A similar strategy is also discussed in J. Wei, F. Liu Macromolecules, 42 (2009), 5486-5491, where 4-[(4-maleimido)thiophenyl]benzophenone was synthesized and polymerized into a macromolecular photoinitiator.
A variety of polymeric photoinitiators other than benzophenone based structures are discussed in T. Corrales, F. Catalina, C. Peinado, N. S. Allen J. Polym. Sci., Part A: Polym. Chem., 159 (2003), 103-114. Polymers with thioxanthone, benzil, anthraquionone, camphorquinone, benzoin ether, acylphosphine oxide and silane functionalites in the macromolecules are discussed in terms of efficiency in comparison to low molecular weight analogues. In some examples, the photoinitiation activity of the polymeric photoinitiator was one order of magnitude higher than the mixture of the corresponding low molecular weight analogues. This increase in activity in the particular case was ascribed to efficient excitation energy transfer between different moieties present on the polymer chain. One other explanation could be the prevention of recombination of radicals formed on the photoinitiator sites as the initiators are “tied” onto a polymer chain.
Some patent literature discusses polymeric photoinitiators. One example is found in US 2007/0078246 where different aromatic ketone systems are substituted on a siloxane polymeric chain. Rates of photopolymerization reactions are then used to prove an enhanced performance of the polymeric photoinitiators as opposed to low molecular weight analogues.
As a further example, benzophenone derivatives with pendant alkyl ether substitutents have been described in WO 96/33156, but the primary properties described were related to migration of the photoinitiators to the surface of a coating. The benzophenones are not repeating units in the polymer and the polymers described in WO 96/33156 can be considered end-functionalized with benzophenone moieties.
A related type of photoinitiator class is described in WO 2009/060235, where thioxanthone moieties are attached to an oligomeric backbone. It is particularly specified that the thioxanthone polymeric photoinitiators have molecular weights below 2000 g/mol.
Several photoinitiators (e.g. benzophenone, anthraquinone) with pendant polyalkyl ethers are described in WO 97/49664. Common to these photoinitiators is that the polyethylene glycol moieties attached to the photoinitiators have preferred molecular weights in the range of 150 to 900 Da and furthermore only one photoinitiator moiety is present per polymeric photoinitiator.
Related to the photoinitiators described in WO 96/33156, similar structures are described in WO 98/51759 where benzophenone derivatives with pendant alkyl ether groups are presented. The main focus of the invention described in WO 98/51759 is oxygen inhibition and low migration properties of the photoinitiator.
WO 03/033492 discloses thioxanthone derivatives attached to a polyhydroxy residue. However, these polyhydroxy groups have only up to 6 hydroxy groups present in the chain.
As a final example of polymeric photoinitiators described in the patent literature, U.S. Pat. No. 4,602,097 details an invention related to water soluble photoinitiators where two photoinitiator moieties are bridged together by a polyalkylether of sufficient length to make it water soluble.
Common to all of the inventions described above from the patent literature, is that the active photoinitiator sites are present as end-groups on for example a polyethylene glycol which falls outside the present invention. The present invention details polymeric photoinitiators in which the photoinitiator moiety itself is an integral part of the repeating unit in the polymeric photoinitiator.